Conventionally, there is known, as one of superconducting devices, a SQUID using Josephson effect. By connecting the SQUID to a magnetic-flux input circuit having a superconducting pick-up coil, there can be obtained a SQUID magnetometer as a kind of magnetic sensor for measuring an extremely faint magnetic field, i.e., a magnetic field generated from faint current in a living body such as a magnetocardiogram, a magnetic field generated from a microscopic magnetic substance in a living body.
When the SQUID magnetometer is cooled to a level of cryogenic temperature, that is, to a temperature level on which the SQUID and the superconducting coil turn to the superconducting state, there may be applied a method of cooling the SQUID magnetometer in such a manner that liquid helium on the level of cryogenic temperature is stored in a cryogenic container (cryostat) and the SQUID magnetometer is steeped in the liquid helium so as to be cooled. In this case, generally, a cooling head of a refrigerator for generating cool condition is entered in the cryogenic container and helium gas evaporated in the container is recondensed into liquid by the refrigerator.
In the above method, since the SQUID magnetometer is steeped in liquid helium, the SQUID magnetometer can be cooled at a short time.
On the other hand, since the cooling of the SQUID magnetometer is carried out by the medium of the liquid helium in the cryogenic container, its cooling system becomes large-sized and its operational performance is deteriorated. In addition, it requires much skill to treat the liquid helium and, depending on circumstances, a careless treatment of the liquid helium may cause a trouble.
Further, since the container in which the liquid helium is stored is not filled to the uppermost end thereof with the liquid helium, the temperature at the inside of the container increases toward the upper part of the container. As a result, a thermal gradient occurs at the inside of the container. This thermal gradient disadvantageously limits an angle capable of inclination of the container. Due to this disadvantage, when a biomagnetic field is measured, it becomes difficult to optionally set the SQUID, the pick-up coil and such in accordance with the condition (posture) of a subject. This is a problem which cannot be disregard.
Therefore, attention has been paid to a conventional method of contacting the SQUID magnetometer with the cooling head of the refrigerator in order that heat can be directly transmitted thereby cooling the SQUID magnetometer (for example, refer to the Japanese Patent Application Laid Open Gazette No. 2-302680).
In this case, the SQUID magnetometer, the pick-up coil and such are attached and thermally connected to a final cooling stage to be cooled below a transition temperature of superconductivity by the cryogenic refrigerator. Accordingly, if only the operation of the cryogenic refrigerator is controlled, selection makes possible between the superconducting state and the normal conducting state. This does not require to move the SQUID, the pick-up coil and such for the above selection.
On the other hand, when the SQUID magnetometer is cooled by the refrigerator in the above way, there is generated the following two problems. The pick-up coil of the magnetic-flux input circuit is generally wound into loops around a tubular bobbin made of resin. However, since the thermal conductivity of the resin forming the bobbin is low, it is very difficult to cool the pick-up coil around the bobbin to its transition temperature of superconductivity when the SQUID magnetometer is directly cooled by the refrigerator as above-mentioned.
It may be possible to form the bobbin by metal such as copper and aluminium with high thermal conductivity even in a range of cryogenic temperature. In this case, however, a normal conducting current loop generates at the bobbin in close vicinity to the magnetic-flux input circuit. As a result, mutual inductance generates between the current loop and the pick-up coil. This invites another problem that output-to-input characteristic of the SQUID magnetometer varies with frequency.
A second problem is described next. There may be another structure in which the SQUID is connected to the cryogenic refrigerator so as to be thermally conductable. For example, in the structure, a superconducting shield member containing the SQUID is placed on the center of the top surface of a stage which is cooled below the transition temperature of superconductivity (for example, a 4 K stage cooled to about 4 K) by the cryogenic refrigerator, a thermally-conductive block member is arranged in such a manner as to interpose and cross over the superconducting shield member, and a bobbin around which a pick-up coil is wound is stood on the center of the top surface of the thermally-conductive block. In this case, since the superconducting shield member is cooled below the transition temperature of superconductivity by the 4 K stage, this prevents the SQUID located inside the shield member from being affected by the outer magnetic field. At set positions on the superconducting shield member, holes are formed for being inserted by an electric wire which connects between the SQUID and the pick-up coil and an electric wire which connects between the SQUID and an electronic circuit arranged on a room temperature's side.
In the above structure, however, it is required to dispose the thermally-conductive block member in such a manner as to cross over the superconducting shield member. Therefore, the plane form of the SQUID magnetometer can be little lessened and it is difficult to make the SQUID magnetometer multi-channelized for high-precise measurement of a biomagnetic field.
Description is made in detail about the above problem. When the SQUID magnetometer is lessened in size, the superconducting shield member, the thermally-conductive block and the bobbin may be lessened in size. However, the bobbin cannot be minimized because its sensitivity of detecting magnetic flux is lowered when the bobbin is lessened in diameter. Further, because the necessary size of the superconducting shield member is determined by the size of the SQUID to be contained therein, the superconducting shield member is limited in its minimization. Consequently, the thermally conductive block arranged in such a manner as to cross over the superconducting shield member is limited in its minimization.
Furthermore, in view of an actual process of producing the SQUID magnetometer, the pick-up coil is wound around the bobbin so as to form first-order or second-order differential type coil. This requires the following producing steps. From this, minimization of the SQUID magnetometer is also limited. The producing steps are as follows: The pick-up coil is wound in such a manner as to be provided together with the thermally-conductive block and the bobbin; a lead wire from the pick-up coil is conducted to the SQUID with the pick-up coil thermally-connected to the bobbin and the thermally-conductive block; and then the lead wire is electrically connected to the SQUID by superconductive soldering or the like. Accordingly, it is required that an inner space of the thermally-conductive block has an area allowing the above work of electrical connection, in detail, an area allowing insertion of a soldering iron and fingers. Since the size of the inner space of the thermally-conductive block is limited as mentioned above, the SQUID magnetometer can be little lessened in its form projected on the plane. Since it is a matter of course that a complicated producing process for the SQUID magnetometer is required, it costs long time to assemble the SQUID magnetometer. As for this point, even in case of the above electric connection between the SQUID and the pick-up coil, the above-mentioned complicated process is required. On the other hand, in case of electrical connection between the SQUID and the electronic circuits on the room temperature's side, a thermal anchor is required for thermal connection of the lead wire to the 4 K stage, and the lead wire requires to be wired to some proper positions and then to be conducted on the room temperature's side. This leads to extremely complicated work as a whole. In addition, depending on the relative positions between the plural SQUID magnetometers, the position for the thermal anchor cannot be ensured.
If the 4 K stage is increased in size, the SQUID magnetometer is relatively lessened in size. However, since the 4 K stage is cooled in a way of thermal conduction by the cryogenic refrigerator, the 4 K stage cannot be increased in size above the cooling capability of the cryogenic refrigerator. This also prevents the SQUID magnetometer from being multi-channelized.
Further, even if the multi-channelization of the SQUID magnetometer is accomplished, there cannot be checked and repaired the SQUID magnetometer which is surrounded by other SQUID magnetometers and has trouble, except after the other SQUID magnetometers are taken apart and removed. This extreme lowers operating performance in maintenance, check and repair of the SQUID magnetometer.
Furthermore, since the thermally-conductive block is generally so composed that plural blocks are fixed in order by screws or the like, the heat resistance of the thermally-conductive block is different between portions thereof depending on the contacting state between the plural blocks. This may make the temperature of the pick-up coil different between one SQUID magnetometer and another SQUID magnetometer, and, in the worst case, a pick-up coil of at least one SQUID magnetometer cannot be cooled below the transition temperature of superconductivity.
In view of the foregoing problems, this invention has been made. A first object of this invention is to improve the structure of a bobbin around which a pick-up coil is wound in a SQUID magnetometer, that is, to enhance the cooling efficiency of the SQUID magnetometer to the pick-up coil without affecting input/output characteristic of the SQUID magnetometer, thereby effectively carrying out the cooling of the SQUID magnetometer by a refrigerator.
A second object of this invention is to readily multi-channelize the SQUID magnetometer which is cooled in a way of thermal conduction by a cryogenic refrigerator, to simplify an assembling work of the SQUID magnetometer, and to extremely enhance the operation performance in maintenance, check and repair of the SQUID magnetometer.